1. Field of the Invention
The present invention relates to hydraulic control devices for working machines such as hydraulic excavators.
2. Description of the Related Art
In hydraulic excavators, for example, some (surplus) oil discharged from pumps is returned (unloaded) to tanks (bleed-off control).
In general, this control is performed by changing the opening areas of bleed-off paths formed in control valves for corresponding actuators in response to inputs through operating means.
However, the length of the control valves is increased due to the length of the bleed-off paths in a spool shaft direction, resulting in an increase in cost and difficulty in assembling the control valves into machines.
To date, in some technologies, a common unified bleed-off valve has been provided for a plurality of control valves (hydraulic actuators) instead of the above-described bleed-off paths for the corresponding control valves (unified bleed-off system).
Moreover, a method for electronically controlling a hydraulic pilot valve serving as the unified bleed-off valve using a downstream pressure of a proportional solenoid valve that is controlled by a controller is often employed in such unified bleed-off systems (for example, Japanese Unexamined Patent Application Publication No. 11-303809).
This unified bleed-off control method has high control flexibility compared with a hydraulic control method in which a pilot pressure according to the operation is directly input to the unified bleed-off valve.
However, in this method, operation of the actuators may be hindered when the unified bleed-off valve fails.
For example, if the unified bleed-off valve that blocks oil when it is in a neutral state fails while all the control valves are in a neutral state (all the actuators are not operated), a relief valve is operated and generates heat.
In contrast, if the unified bleed-off valve that unloads oil when it is in a neutral state fails, the unloading state of the unified bleed-off valve is maintained. Thus, the actuators become inoperable, and the machine stops moving.
In particular, in the above-described electronically controlled system, these problems are serious since the proportional solenoid valve often breaks down or the unified bleed-off valve often fails due to abnormalities such as breaks in wires that transmit control signals from the controller to the proportional solenoid valve in the control system.
On the other hand, in hydraulic excavators, hydraulic actuators are divided into two groups, and the two groups are driven by separate pumps such that the total flow rate of the pumps is efficiently divided and distributed to the actuators in view of flow rates required for the hydraulic actuators, combined control, and the like.
In this case, left and right driving motors (hydraulic motors) that drive crawler-mounted traveling sections belong to the separate groups, and are basically driven by the separate pumps.
For example, in the case of the structure where oil discharged from two hydraulic pumps is distributed to two groups, it is preferable that the oil discharged from both pumps be merged and distributed to both groups for ensuring a required flow rate when control valves for both the driving motor and an attachment in the first group are operated at the same time (hereinafter referred to as a simultaneous operation).
Therefore, a switching valve for switching flow channels is disposed at the discharge side of the pumps such that the oil discharged from both pumps is merged and distributed to both groups during the simultaneous operation.
Accordingly, when the unified bleed-off control method is employed, the problems that occur during failure of the unified bleed-off valve must be solved with consideration of the above-described circuit structure.